The present invention is in the field of electrical power generation and distribution systems and, more particularly, systems which may be employed in aerospace vehicles.
In a typical prior-art aerospace vehicle such as an aircraft, there may be many different requirements for electrical power. Many functions may be performed with electrical motors and controls.
On-board generators may be driven with various prime movers such as turbine engines. In many cases, a prime mover may drive a generator only as an ancillary function. A typical primary function for a prime mover, such as a turbine engine, may be to provide propulsion thrust for the aircraft. In the context of its primary function, the prime mover may operate at varying rotational speeds. A generator coupled to a shaft of such a variable-speed prime mover may rotate at varying speeds.
As aircraft designs evolve, more of the ancillary power requirements are being met with electrical systems instead of previously used bleed air and hydraulic systems. An evolving design concept has become known as “more electric aircraft” (MEA). In the context of MEA designs, electrical loads on generators may be become quite large. Indeed, a generator load may become large enough to negatively affect engine thrust output. Because of these increased electrical power demands in MEA design, a single generator driven by a single prime mover may not be capable of producing all of the electrical power for an aircraft. Consequently, an aircraft may be provided with multiple generators, each driven by different prime movers.
Because prime movers have varying rotational speed during operation of the aircraft, rotational speed of any particular generator may differ from rotational speed of other generators on the aircraft. In the case of alternating current (AC) generators, each AC generator may produce AC power at a frequency and phase angle different from the other AC generators. It may be said that, each generator may produce “variable frequency” electrical power.
Certain aircraft operating conditions may arise in which a particular generator may be subjected to a particularly high load demand during a time when its associated prime mover may be performing its primary function (e.g. producing thrust) at a relatively low speed. In order to meet the high electrical power requirement of an attached generator, it may be necessary to increase the speed of the prime mover, even though such an increase in speed may not otherwise be required for the primary function of the prime mover.
Excessive fuel may be consumed if and when a prime mover is operated at a speed greater than required for its primary role. Certain design efforts have been directed to this issue. For example U.S. Pat. No. 7,285,871 (Jean Luc Derouineau) issued Oct. 23, 2007, discloses multiple generators that may be driven on different shafts of a turbine machine. The turbine machine may have a low-pressure turbine output shaft and a high-pressure turbine output shaft. A separate generator may be driven by each of the shafts. Electrical outputs of the generators may be shared and controlled so that electrical loads may be allocated to either the low-pressure turbine or the high-pressure turbine as a function of turbine operating speed. This allocation may facilitate efficient operation of the turbine machine.
This prior-art power allocation method may require paralleling of two or more AC generators onto a common power bus. Successful paralleling of AC generator outputs may require matching of frequency of the generators. Thus this prior-art method, when employed with AC generators, may be practical only when the AC generators operate at the same rotational speed. Alternatively, as in well understood prior art, the generators may be driven via a constant speed transmission to match their frequency and phase, or may use power electronics to synthesize a matched AC output. Both of these techniques require large, complex and expensive devices to facilitate paralleling.
Many MEA aircraft employ multiple turbines that may operate at different speeds. Each of the turbines may drive AC generators. It has heretofore not been practical to allocate electrical power requirements of multiple-engine aircraft to all of the generators of the aircraft as required by the operational conditions.
As can be seen, there is a need to provide power generation and distribution systems in which AC power produced by multiple generators operating at different speeds may be paralleled to a common bus. Additionally, there is a need to provide such a system in which electrical loads may be allocated to any prime mover of a multiple-engine aircraft, or any turbine of a multiple-turbine prime mover.